Ladder length calculator
This ladder length calculator finds the ladder you need for a given height. OSHA says the base should be about one quarter of the working length out from the support – a 75.5° angle. To reach a 12 ft roof edge with 3 ft of overhang you need about 15.5 ft of ladder, set 3.1 ft from the wall.
Example: Calculate: Ladder length for a height · Unit: feet · Height of the support point 12 · Ladder angle 75.5 ° · Extension above the support 3 → Result: Ladder length at least 15.5 ft, base 3.1 ft from the wall.. Source: OSHA 29 CFR 1926.1053(b)(1) side rails at least 3 feet (.9 m) above the upper landing surface; (b)(5)(i) horizontal distance approximately one-quarter of the working length of the ladder (read 10.10.2026). Updated: .
How it is calculated
Ladder length = (height + extension) ÷ sin(angle); base distance = height ÷ tan(angle). OSHA rule 29 CFR 1926.1053(b)(5)(i) says non-self-supporting ladders shall be used at an angle where the horizontal distance from the top support to the foot is approximately one quarter of the working length – the 4-to-1 rule, which is an angle of 75.5°. Rule 1926.1053(b)(1) adds that the side rails shall extend at least 3 feet above the upper landing surface when the ladder is used for access.
Ladder length needed (4-to-1 rule, 3 ft extension)
| Height of the support | Ladder length | Base distance |
|---|---|---|
| 8 ft | 11.4 ft | 2.1 ft |
| 10 ft | 13.4 ft | 2.6 ft |
| 12 ft | 15.5 ft | 3.1 ft |
| 16 ft | 19.6 ft | 4.1 ft |
Without the extension, a 12 ft support height needs 12.39 ft of ladder at 75.5°. The calculator treats the 3 ft as measured vertically above the landing, which is slightly more ladder (3.1 ft along the rails) – the cautious choice.
Check your setup
- Measure the ladder length to the support and the base distance; 16 ft of ladder with the base 4 ft out is exactly one quarter, 75.5°.
- If the base is farther out, the angle is flatter and the foot can slide; if closer, the ladder can tip backward.
For right triangles in general, use the Pythagorean theorem calculator; for roof jobs, the roof area calculator.
Frequently asked questions
What is the 4-to-1 ladder rule?
For every 4 feet of ladder length to the support, the base sits about 1 foot out from the wall. OSHA 1926.1053(b)(5)(i) calls it approximately one quarter of the working length; that is an angle of 75.5°.
What size ladder do I need for a 12 ft roof?
With the 4-to-1 angle and 3 ft of extension above the roof edge, about 15.5 ft of ladder, with the base 3.1 ft from the wall.
How far should a ladder extend above the roof?
OSHA 1926.1053(b)(1): at least 3 feet above the upper landing surface when the ladder is used for access.
How far should the base of a 16 ft ladder be from the wall?
About 4 ft, one quarter of the working length.
How do I find the angle of my ladder?
Enter the length to the support and the base distance in “Check the angle”: the angle is arccos(base ÷ length).
Sources and legal basis
As of: · Editorial policy
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